In 3GPP (3rd Generation Partnership Project), which is a project aiming to standardize a mobile communication system, attempts are being made to enhance LTE (Long Term Evolution) to comply with rapidly increasing traffic demands. As one of such attempts, it has been discussed to use, for LTE communication, not only a frequency band for which a license is granted to an operator (licensed band) but also a frequency band for which a license is not required (unlicensed band).
In the unlicensed band, in order to avoid interference with another system different from an LTE system (such as wireless LAN) or an LTE system of another operator, a listen-before-talk (LBT) procedure is requested. The LBT procedure is a procedure to monitor a frequency in the unlicensed band to check, based on a received power (interference power), whether or not the frequency is available, and only if the frequency is confirmed to be available (clear channel), the frequency is used.